The present invention relates to electroluminescent phosphor particles, particularly, to phosphor particles which are encapsulated in a moisture resistant coating and exhibit high electroluminescent brightness and, even more particularly to such an electroluminescent phosphor particle encapsulated with a metal oxynitride protective coating having improved electrical, chemical, thermal-mechanical, or surface characteristics. The present invention also relates to a method for making such encapsulated phosphor particles.
Phosphor particles are used in a variety of applications such as flat panel displays and decorations, cathode ray tubes, and fluorescent lighting fixtures. Luminescence or light emission by phosphor particles may be stimulated by application of various forms of energy including electric fields (electroluminescence). Electroluminescent (xe2x80x9cELxe2x80x9d) phosphors have significant commercial importance. The luminescent brightness of such phosphors and the xe2x80x9cmaintenancexe2x80x9d of this, brightness are two criteria typically used to characterize phosphor particles.
Luminescent brightness is typically reported as a quantity of light emitted by the subject phosphor when excited. Because of the sensitivity of phosphor emission brightness to varying conditions of excitement, it is often useful to report the brightness of phosphors as relative brightness rather than as absolute brightness. xe2x80x9cMaintenancexe2x80x9d refers to the rate at which phosphors lose brightness (i.e., decay) with operating time. The rate of decay is substantially increased if the phosphor particles are subjected to conditions of high humidity while being operated. This effect of moisture or high humidity has been referred to as xe2x80x9chumidity-accelerated decayxe2x80x9d.
Particulate EL phosphors are most commonly used in thick film constructions. These devices typically include a layer of an organic material. having a high dielectric constant which forms a matrix for a load of phosphor particles. Such layers are typically coated on a plastic substrate having a transparent front electrode. A rear electrode is typically applied to the backside of the phosphor layer, with a dielectric layer sandwiched there between. When an electric field is applied across the electrodes, the proximate portions of the layer emit light as the phosphor particles therein are excited.
Organic matrices and substrate materials, as well as organic coatings applied to individual particles, have typically been ineffective in preventing the decay of brightness caused by the diffusion of water vapor to the phosphor particles. For this reason, thick film electroluminescent devices have been encased in relatively thick envelopes, e.g., 25 to 125 microns, of moisture-resistant polymeric materials. However, such envelopes are typically expensive, result in unlit borders, and have the potential of delaminating, for example, under heat.
To improve their moisture resistance, phosphor particles have been encapsulated in an inorganic coating, such as a coating of a metal oxide. Inorganic coating techniques have been employed with varying degrees of success. Hydrolysis-based processes for encapsulating EL phosphor particles in an inorganic coating, e.g., hydrolysis-based chemical vapor deposition (CVD), have typically been the most successful. In hydrolysis-based CVD processes, water and oxide precursors are used to form the protective coating. Such hydrolysis-based CVD processes have been able to produce moisture insensitive encapsulated phosphor particles, while minimizing process related phosphor damage and retaining a high initial luminescent brightness.
Thus, existing coatings on phosphors provide protection against humidity accelerated decay, but there is a continuing need for new coating materials which provide protection from moisture, and which provide improved electrical, chemical, thermal-mechanical, or surface characteristics.
The present invention provides novel encapsulated phosphor particles, each having a substantially transparent metal oxynitride coating. The encapsulated phosphors exhibit a reduced sensitivity to humidity-accelerated decay of luminescent brightness. Additionally, the present invention involves a method which results in the encapsulated particles.
Each encapsulated particle of the present invention includes a phosphor particle of an electroluminescent phosphor material which exhibits humidity-accelerated decay in the presence of moisture. The phosphor particle is coated with a substantially transparent metal oxynitride. The metal oxynitride sufficiently encapsulates the phosphor particle to provide the particle with reduced sensitivity to humidity-accelerated decay.
The phosphor particles are generally made of at least one of a zinc sulfide-based phosphor, a calcium sulfide-based phosphor, a zinc selenide-based phosphor, a strontium sulfide-based phosphor or a combination of the phosphor compounds. The phosphor particles are sensitive to humidity-accelerated decay and to thermal degradation if exposed to high; temperatures.
In accordance with the present invention, a metal oxynitride coating sufficiently encapsulates the phosphor particle to limit exposure of the phosphor to moisture or water. The metal oxynitride coating includes one or more layers of a single metal oxynitride, a mixed metal oxynitride, or a combination of such layers. The one or more layers are generally applied such that the total thickness of the metal oxynitride coating is in the range of about 0.03 microns to about 1.0 microns. The metal component in the metal oxynitride is preferably selected from aluminum, boron, silicon, titanium, zirconium, or a combination of the preferred metals. Preferably, the oxynitride coating of the present invention has a nitrogen to oxygen molar ratio in the range of about 4:1 to about 1:4.
The metal oxynitride coating of the present invention exhibits reduced sensitivity to chemical degradation caused by exposure to condensed moisture or otherwise liquid water (i.e., greater resistance to corrosion in a liquid water environment). It is desirable for the present metal oxynitride coating to be sufficiently non-porous. The non-porous coating provides a phosphor particle that exhibits reduced sensitivity to humidity. Preferably, the coating is sufficiently non-porous and is also sufficiently resistant to chemical degradation (i.e., corrosion) from water such that the encapsulated particle can survive immersion in a 0.1 molar silver nitrate aqueous solution, with substantial resistance to darkening. Such a silver nitrate test has typically been used to check the permeability of a phosphor coating. Being more resistant to water induced corrosion enables the present metal oxynitride coating to survive for longer periods in a liquid water environment. Additionally, the encapsulated particle of the present invention preferably has an initial electroluminescent brightness of about 50% or greater of the initial electroluminescent brightness of the phosphor particle with no coating.
The present invention also provides a novel method for making such encapsulated phosphor particles. The method comprises providing a bed of phosphor particles, each of which exhibits humidity-accelerated decay in the presence of moisture; providing one or more precursors comprising a vapor phase metal containing precursor, a vapor phase nitrogen containing precursor, and a vapor phase oxygen containing precursor; and exposing the bed to the precursors such that the precursors chemically react and encapsulate each phosphor particle with a metal oxynitride coating. The one or more precursors utilized in the present invention could include compounds in which the metal component and the nitrogen component are present in a single precursor. Additionally, a single precursor containing the metal component, nitrogen and oxygen could be utilized to form the metal oxynitride coating of the present invention. The resulting coating is substantially transparent and sufficiently encapsulating to provide the phosphor particle with reduced sensitivity to humidity-accelerated decay.
In accordance with the present invention, it has been discovered that a metal oxynitride coating may be applied onto a phosphor particle to protect the particle from humidity-accelerated decay.
The particles utilized in the present invention are generally phosphor particles which exhibit luminescence or light emission upon stimulation by an electrical field. An electroluminescent phosphor particle of the present invention can comprise, for example, a zinc sulfide-based phosphor, a calcium sulfide-based phosphor, a zinc selenide-based phosphor, strontium sulfide-based phosphor or combinations thereof. Phosphors used in the present invention may be formulated in accordance with conventional practices. For example, zinc sulfide based phosphors are well-known and commonly include one or more of such compounds as copper sulfide, zinc selenide, and cadmium sulfide in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. Phosphor particles used herein may be of many sizes, typically depending to a large extent on the desired application. Each phosphor particle utilized in the present invention demonstrates the undesirable characteristic of accelerated decay when exposed to moisture or water.
A metal oxynitride coating is utilized to protect the phosphor particles from humidity-accelerated decay. As used herein, a metal oxynitride coating refers to a material made up primarily of at least one metal, nitrogen and oxygen. For purposes of the invention, the coating is defined as one or more layers of a single metal oxynitride, a mixed metal oxynitride, or a combination of such layers. The coating is substantially transparent to permit the passing of light emitted from the phosphor. Additionally, the coating is sufficiently non-porous to adequately encapsulate the phosphor particle and reduce its sensitivity to humidity-accelerated decay. The one or more layers are generally applied such that the total thickness of the metal oxynitride coating is in the range of about 0.03 microns to about 1.0 microns. Coatings which are too thick may tend to be less transparent and result in reduced brightness.
The metal component of the oxynitride coating is preferably selected from the group consisting of aluminum, boron, silicon, titanium, and zirconium or combinations thereof. The molar ratio of components in the oxynitride coating indicates that nitrogen is included in the coating at levels exceeding trace amounts and thus sufficient to enhance or improve the electrical, chemical, thermal-mechanical, or surface characteristics of the coating over an essentially nitrogen-free metal oxide coating. The oxygen content of the coating exceeds trace levels that may be present in pure nitride coatings. In general, the oxygen component of the oxynitride coating provides transparent properties to enable the transmission of visible light and to improve the desired physical properties. The transparency of the metal oxynitride coating is sufficient to enable a useful level of visible light from the phosphor to pass through the coating. The nitrogen to oxygen molar ratio of the oxynitride coating is preferably in the range of about 4:1 to about 1:4, most preferably about 3 or 2:1 to about 1:2 or 3, and even more preferably near about 1:1. The molar ratio of the metal component in the metal oxynitride coating may vary significantly due to the valence of the particular metal used in the coating. The oxynitride may also contain amounts of other elements and compounds, including those originating in the precursor materials or phosphor particles, which can be generated in coating form on phosphor particles under conditions that are at least similar to that described herein.
The method of the present invention comprises: providing a bed of phosphor particles, each of which exhibits humidity-accelerated decay in the presence of moisture; providing one or more precursors comprising a vapor phase metal containing precursor, a vapor phase nitrogen containing precursor; and a vapor phase oxygen containing precursor; exposing the bed to the precursors such that the precursors chemically react and encapsulate the phosphor particles with a metal oxynitride coating. The one or more precursors include any suitable precursor capable of forming the desired metal oxynitride for the coating. The resulting coating is substantially transparent, resistant to chemical degradation from moisture and water, and sufficiently encapsulating to provide the phosphor particle with reduced sensitivity to humidity-accelerated decay.
The method of the present invention is generally practiced through the use of conventional chemical vapor deposition (CVD) techniques. For example, the metal oxynitride coating could be applied onto the phosphor particles utilizing atmospheric pressure CVD or plasma enhanced CVD. Alternatively, other conventional coating practices, such as sputtering, may be used in applying the metal oxynitride coating. The coating process includes exposing the bed of phosphor particles to the precursor gas mixture so as to coat each phosphor particle by a vapor phase reaction of the one or more precursors including a vapor phase metal containing precursor, a nitrogen containing precursor, and an oxygen containing precursor. The reaction occurs at a temperature under conditions which at least substantially minimize thermal-chemical related damage to the phosphor particles being encapsulated.
In practicing the method of the present invention, uncoated phosphor particles are placed in a reactor and heated to the appropriate temperature. In order to form coatings which sufficiently encapsulate the phosphor particles, the particles are preferably agitated while in the reaction chamber. Illustrative examples of useful methods for agitating the phosphor particles include shaking, vibrating, or rotating the reactor, stirring the particles, or suspending them in a fluidized bed. In such reaction chambers, the particles may be agitated by many different ways such that essentially the entire surface of each particle is exposed and the particles and reaction precursors may be well intermixed. Typically, a preferred reaction chamber is a fluidized bed reactor. Fluidizing typically tends to effectively prevent agglomeration of the particles, achieve uniform mixing of the particles and reaction precursor materials, and provide more uniform reaction conditions, thereby resulting in highly uniform encapsulation characteristics.
Although not required in many instances, when using phosphor particles which tend to agglomerate, it may be desired to add fluidizing aids, e.g., small amounts of fumed silica. Selection of such aids and of useful amounts thereof may be readily determined by those with ordinary skill in the art.
The desired precursor materials in vapor phase are then added to the reactor wherein a vapor phase reaction occurs to form a coating of a metal oxynitride on the surfaces of the phosphor particles. The vapor phase metal containing precursor is generally a metal compound that is capable of reacting with other precursor gas streams to provide the metal component of the metal oxynitride coating. Metal chlorides, for example, are typically utilized in CVD process as a source of metal. Additionally, organic metal sources may be utilized in producing the coating of the present invention. For example, alkyl silanes may be used to provide a source for silicon for the metal oxynitride coating. Examples of other volatile metal containing precursors compounds include metal alkyls, metal alkoxides, metal carbonyls, and metal diketonates. More than one metal precursor may also be used in order to form the mixed metal oxynitride coatings of the present invention.
The nitrogen containing precursor of the present invention is generally any nitrogen compound that is capable of reacting with the other vapor phase precursor streams to form the desired coating. Preferably, ammonia is utilized as the nitrogen containing precursor when separate metal containing and oxygen containing precursors are utilized. The oxygen containing precursor is generally oxygen or water. However, other conventional sources of oxygen suitable for CVD applications may be used in accordance with the present invention.
Alternatively, the metal source and nitrogen source may be provided in at least a single compound. The use of compounds having a metal-nitrogen bond enable lower temperature CVD reactions without the use of plasma, thus reducing thermal-chemical degradation of the phosphor particles. An example of compounds having a metal-nitrogen bond are methyl amino complexes (for example, tetrakis dimethyl amino titanium) of the preferred metals of aluminum, boron, silicon, titanium, or zirconium. These compounds may be utilized as the both the source of the metal and nitrogen, or may be used in conjunction with other metal containing precursors or nitrogen containing precursors to form the metal oxynitride coating.
The metal oxynitride coating of the present invention may also be produced through the use of a single source precursor which includes a metal, nitrogen, and oxygen. For example, a vapor phase precursor containing a metal, alkyl amino ligands, and oxygen containing ligands, such as alkoxy or carboxylate groups, could be used in the inventive process.
The precursor streams are directed to the reaction chamber in the vapor phase. One technique for getting the precursor materials into vapor phase and adding them to the reaction chamber is to bubble a stream of gas, preferably inert, referred to herein as a carrier gas, through a neat liquid of the precursor material and then into the reaction chamber. Illustrative examples of inert gases which may be used herein include argon and nitrogen. Oxygen and/or air may also be used, provided the desired N/O ratio can still be maintained. However, the introduction of excessive amounts of oxygen into the reactor can prevent the formation of the desired oxynitride coating with the requisite nitrogen content. Additionally, it may be necessary to rely on back diffusion, with an open reactor, or to rely on a nitrogen purge to obtain the desired oxynitride coating. An advantage of utilizing a carrier gas is that the carrier gas/precursor streams may be used to fluidize the phosphor particles in the reaction chamber, thereby facilitating the desired encapsulation process. In addition, such a technique provides means for readily controlling the rate of introduction of the precursor materials into the reactor.
The precursor gas streams are exposed to the phosphor compounds where they react and form the metal oxynitride coating of the present invention. When forming the coating, all of the streams are transported into the reactor at the same time. When forming a layered metal oxynitride coating, precursor streams for the initial layer are first transported into the reactor until the particles are encapsulated. Subsequent layers are then formed by directing additional precursor streams onto the encapsulated particles. It may be desirable for the outer layer to comprise a metal oxide coating over the metal oxynitride coating.
Precursor flow rates are adjusted to provide an adequate deposition rate and to provide a metal oxynitride coating of desired quality and character. Flow rates are adjusted such that the ratios of precursor materials present in the reactor chamber promote oxynitride deposition at the surface of the phosphor particles.
Optimum flow rates for a particular application typically depend in part upon the temperature within the reaction chamber, the temperature of the precursor streams, the degree of particle agitation within the reaction chamber, and the particular precursors being used. Those skilled in the art are capable of establishing useful flow rates through experimentation. It is desirable for the flow rate of the carrier gas used to transport the precursor materials to the reaction chamber to be sufficient to agitate the phosphor particles as desired and also transport optimal quantities of precursor materials to the chamber.
It is also desirable for the precursor materials to have sufficiently high vapor pressures so that large enough quantities of precursor material will be transported into the reactor for the coating process to proceed at a conveniently fast rate. For instance, precursor materials having higher vapor pressures will typically provide faster deposition rates than will precursor materials having lower vapor pressures, thereby enabling the use of shorter encapsulation times. Precursor sources may be heated to increase the vapor pressure of the material. In order to prevent condensation between the heated source and the reactor, it may be necessary to heat the tubing or other means used to transport the precursor material to the reactor. In many instances, like, those found tabulated below, the precursor materials will be in the form of neat liquids at room temperature. In some instances, the precursor materials may be available as solids which can be made to sublime.
The precursor materials that are the most desirable are those that are capable of forming the present coatings at temperatures that are low enough not to cause substantial damage to the phosphor particles. It is desirable for the temperature of the reactor to be maintained at temperatures which help insure that the coatings being deposited are sufficiently encapsulating to provide desired protection against humidity-accelerated decay, and are resistant to corrosion from liquid water, while avoiding intrinsic thermal damage or adverse thermal-chemical reactions at the surfaces of the particles which cause undesirable loss of initial brightness. Temperatures required to form oxynitride compounds in CVD reactions tend to be higher than certain reactions to form oxides. However, conditions tend to be less strongly oxidizing and reactive precursors, or precursors containing metal-nitrogen bonds, can be employed as exemplified herein, so as to reduce or minimize thermal-chemical damage to the phosphor particles. Encapsulation processes which are performed at temperatures which are too low may tend to result in coatings which do not provide the desired resistance to humidity-accelerated decay. Such coatings are not sufficiently moisture impermeable because, it is believed, of having a more open structure or a structure which contains excess trapped or unreacted precursor components. Encapsulation processes which are performed at temperatures which are too high may result, for example, in decreased electroluminescent brightness, undesirable changes or shifts in the color of the light emitted by the subject phosphor, or degradation of the intrinsic decay characteristics of the subject phosphor material.
The encapsulated phosphor particles of the present invention provide both reduced sensitivity to humidity and improved electrical, chemical, thermal-mechanical, or surface characteristics. The resulting coatings of the present invention are sufficiently non-porous to provide a phosphor particle with a substantial resistance to darkening when the encapsulated particle is exposed to silver nitrate. Additionally, the encapsulated phosphor particles retain an acceptable level of their initial luminescent brightness. Preferably, each of the encapsulated particles has an initial electroluminescent brightness which is equal to or greater than about 50 percent of the initial electroluminescent brightness of the phosphor particle. Most preferably, the initial electroluminescent brightness of the encapsulated particle is equal to or greater than about 80 percent of the initial electroluminescent brightness of the phosphor particle.